The present invention is directed to a modulating means for an RF power amplifier having at least one RF power transistor.
In pulse power amplifiers having RF power transistors, pulse shaping corresponding to a prescribed envelope should be undertaken during the amplification of RF pulses to a high output power and the peak pulse power should achieve a desired value.
One of the critical disadvantages of RF power transistors is non-linear distortion of the pulse shape of the output power resulting from a pulsed mode. For example, this problem is known from German Patent 29 04 011 (corresponding to U.S. Pat. No. 4,322,689 hereby incorporated by reference).
Generating transmission pulses having a defined envelope basically represents an amplitude modulation. It is disclosed in German Patent 28 35 751 (corresponding to U.S. Pat. No. 4,286,236 hereby incorporated by reference) that the known teaching for the control of the modulation signal cannot be transferred without further processing to RF power amplifiers wherein the RF carrier oscillation is modulated with pulses. This is particularly true when the pulses have an extremely short pulse duration, for example 8 .mu.sec. The time required in the prior art controls is too long for these applications.
It is well known that linearity in the amplification can be improved by using a negative feedback. The article by J. Stammelbach, "Probleme bei der Amplitudenmodulation von Transistorleistungsstufen" in the German periodical "Frequenz", Vol. 30, No. 6, Jun. 1976, pages 136-144 discloses a solution for linearization of the pulse shape in RF power amplifiers that is referred to as envelope negative feedback. This linearization possibility is explained with reference to FIG. 1. An RF input signal 7 is supplied to an RF amplifier 2. The part of the RF output pulse 15 branched off in an output means 3 and demodulated with a demodulator 4 is compared in a comparator 5, as actual pulse 8, to a rated pulse 9 that is supplied by a pulse generator 1. Any deviation is eliminated on the basis of a corresponding control of the modulation control voltage 10, except for a residual deviation that is dependent on the controlled gain of an evaluation circuit 6.
The aforementioned German Patent 28 35 751 discloses an RF power amplifier wherein a part of the modulated amplifier output signal is taken, rectified and compared to the rated modulation signal and wherein the actual modulation signal is pre-distorted in accordance with the result of the comparison such that the amplifier output signal has the desired curve.
It is disclosed in German Patent 32 21 911 that a great length of time is required in the known RF power amplifier for matching the actual pulse to the rated pulse if the actual signal deviates too greatly from the rated signal at the beginning of the correction. This is frequently the case when a switch is undertaken from a first to a second carrier frequency.
German Patent 32 91 911 discloses an RF power amplifier in detail, whereby the correction is implemented in the transmission pauses by using intermediate memories. In this case, the control loop is interrupted during the correction. Given the transmitter disclosed therein, a switch is undertaken after the transmission event from the operating mode of "transmitting" to the operating mode of "correcting". It is thereby assumed that the transmission signal has relatively long separations between the transmission pulses or between the pulse groups. This is the case in the distance measuring system DME or the navigation system TACAN.
In certain applications of RF power amplifiers, the pulse sequence is composed of 100 and more pulses per message, whereby the possibility exists of switching to a different carrier frequency and from a first to a second pulse format between two pulses. In these cases, time considerations make it impossible to suitably pre-distort the modulation signal using the disclosed amplifier since all rated and actual amplitude values of all pulses of a pulse group would have been compared to one another for correction.
Previously cited German Patent 29 04 011 discloses a radio-frequency pulse amplifier wherein an amplifier chain is controlled to a constant output power with an AGC control loop. Each of the transistor amplifier stages has a constant current source allocated to it. The control pulses for all constant current sources proceed via a common pulse shaping circuit and are set with respect to their voltage amplitude, namely by an AGC control voltage that is acquired dependent on the total actual output power of the amplifier stages in comparison to the rated output power to be held constant. The signal edges of the pulses output by the pulse shaping circuit that control the constant current sources align the signal edges of the radio-frequency pulses traversing the amplifier stages and can be structured based on certain requirements.
The AGC control loop thereby provided controls the output power of the amplifier stages dependent on the amplitude change of the envelope. The time constant in an AGC circuit is always selected to be of such length that an average DC voltage is formed from the successive radio-frequency pulses, the value of this average DC voltage being proportional to the amplitude of the RF pulses. This problem is discussed in German Patent 12 97 903. The envelope of the radio-frequency pulses can be acquired from the amplifier output signal on the basis of a corresponding selection of the time constant of the rectifier circuit.
The article, "Solid State Amplifiers for DME-Beacons" by D. Graziani in the periodical "Electrical Communication", Vol. 50, No. 4, 1975, pages 273-277 discloses a transistor power amplifier for DME applications wherein the modulation control pulse has a "pedestal". The reason for this modulation technique lies in the initial non-linearity of the control characteristic between the output power and the modulation pulse amplitude. The non-linearity at low modulation voltage effects a change of the pulse shape due to an abbreviation of the rise and decay times. The modulation technique recited therein on the basis of pre-distortion of the modulation control pulse has the disadvantage of being a pure control operation. Thermally conditioned or frequency-dependent changes of the amplifier characteristic during the pulse transmission influence the pulse shape and the output power in an undesirable manner. It is conceivable to acquire the non-linear amplifier characteristic in terms of its temperature and frequency dependency and to take this into consideration when shaping the modulation control pulse. In practice, deviations from the ideal shape of the modulation control pulses already result due to the measuring error in the identification of the non-linear amplifier characteristic or due to unit differences or due to a degradation during operation caused by aging effects. Influencing the modulation control signal dependent on the amplifier output signal could only be achieved on the basis of a control event in the form of negative feedback.
Pulse power amplifiers that generate pulse peak powers of 200 W and above into the GHz range use RF power transistors for pulsed operation. The transistors operate in the non-linear C-mode and are driven into the A-mode and B-mode only during keying (German Patent 29 04 011). Given such amplifiers, there is a non-linear characteristic between the modulation control voltage and the pulse output power that is output. The output power is zero below a "starting voltage" of the amplifier, then increases, for example, according to a quadratic function and reaches a saturation value at a defined modulation control voltage. A non-linear characteristic also results for the RF pulse amplifier recited in German Patent 29 04 011 that has a control on the basis of constant current sources, namely when a constant current source cannot be allocated to each of the transistor amplifier stages. For example, this is the case when minimum mechanical dimensions are required or when the output stage transistors require more than 10 A or when the dissipated power converted in the constant current sources cannot be reliably eliminated due to a pulse-duty factor of 20%.
A pulse shaping with a control loop corresponding to FIG. 1 is fundamentally impossible in such RF power amplifiers having non-linear control characteristic. As a consequence of the pronounced non-linearity at low modulation voltages, a pronounced, non-linear distortion of the output pulse shape occurs not only in the "pulse foot". In particular, the output power, as a function of the time, is zero until the modulation control voltage has reached a value of the "starting voltage" of the amplifier. This effect causes a chronological delay between the leading edges of the modulation control pulse and of the output pulse. The dead time of the amplifier resulting therefrom becomes all the longer the more slowly the modulation control voltage rises as a function of the time. Given pulse shapes having rise times on the order of magnitude of 1 .mu.sec, abbreviations of the output pulse shape therefore occur that also cannot be corrected by an arbitrarily fast control algorithm since the acquired controlled quantity is zero during the dead time.